This invention relates to shield wires employed, for instance, in a circuit (such as that between a microphone and an amplifier) for precisely transmitting variations of electrical current.
A conventional shield wire comprises either a solid or stranded-wire inner conductor of copper alloy or the like surrounded by an electrical insulating material or a dielectric material such as vinyl or the like. A braid of copper alloy or the like is woven over the dielectric material to form the outer conductor, and a covering of elastic dielectric such as synthetic rubber is placed on top of the braid.
With the conventional shield wire, it is possible to reduce the influence of induction hum due to external magnetic fields to some extent with the aid of the braid. However, in the case when the shield wire is extremely bent during the operation of an electrical device provided with the shield wire, it is difficult to precisely transmit the variations of electrical current therethrough. For instance, it is well known in the art that when a cable connected to a microphone is extremely bent during operation, noise signals are generated.
A cause of this effect is considered to be that at the moment when the shield wire is bent, static electricity is generated in the braid and accordingly the electrostatic capacitance between the braid and the inner conductor is changed, as a result of which electrical current corresponding to the change of the electrostatic capacitance is caused to flow in the circuit, which leads to the generation of noise signals.